It is known to define and correlate the position of single or multiple points within the cranium with preoperative imaging by using frameless stereotactic systems such as the MAYFIELD/ACCISS Stereotactic Workstation which is commercially available from the assignee of this application. Frameless systems provide constant intraoperative navigational information, which permits a surgeon to identify precisely the spacial position of a probe in the surgical field with CT or MRI scan data shown on a high definition display monitor.
With a frameless stereotactic system, an intracranial target point is accessed by advancing a probe along a predetermined linear path or trajectory to the target point within the patient's skull. To provide the necessary stabilization of tooling during its advance along the linear trajectory, a surgical instrument support is used. The surgical instrument support is comprised of an articulated arm having a proximal end mounted on a patient support or other fixed structure and a lockable tool socket rotatably mounted on a distal end of the arm. The surgical instrument support allows a surgical instrument to be moved to the intracranial target point along a stable and fixed linear trajectory. By fixing the trajectory on the intracranial target point, the risk of misdirection or drift associated with freehand procedures is eliminated.
One example of a known surgical instrument support is the "EASYGUIDE" navigator system commercially available from Phillips Medical Systems N.A. Inc. of Shelton, Connecticut. Another example of a surgical instrument support is commercially available from the assignee of the present invention. Other examples of known surgical instrument supports are shown in U.S. Pat. Nos. 5,695,501 and 5,810,712, which are assigned to the assignee of this invention and hereby expressly incorporated by reference herein. All of these devices include a lockable ball rotatably mounted in a tool holder on the end of an articulated arm as described above. The ball has a diametric hole that receives an instrument. In a known manner, a known locating probe is inserted into the ball with the tip of the probe normally being positioned substantially at the center of the ball. The probe presents a linear image on a display monitor that is also displaying CT or MRI scan data of the patient. Thus, as the probe is moved, a path between the tip of the probe and a selected target point displayed with the scan data can be tracked. By moving the distal end of the articulated arm, the probe and ball are first located at a desired position with respect to the skull that defines a desired trajectory between the tip of the probe and the target point. The articulated arm is then locked, thereby locking the ball at the desired position. Next, the probe and ball are rotated to align a centerline of the hole in the ball with the desired trajectory, so that an instrument inserted through the hole in the ball follows the desired trajectory and intersects the target point. The ball is then locked in place, so that it cannot rotate with respect to the tool holder on the distal end of the articulated arm; and hence the orientation of the probe or other tool within the ball is fixed with respect to the target point. With the ball thus aligned, the probe is removed and other surgical instruments inserted into the ball are automatically aligned with the intracranial target point.
While these prior art devices have proved suitable for their intended purposes, they all have one particular disadvantage. In the above described process, after the surgeon has established the desired trajectory by locking the articulated arm and ball in place, the articulated arm and ball can interfere with procedures that are being conducted within the surgical field that do not require the presence of the articulated arm and ball. Thus, it is often desirable and sometimes necessary to move the articulated arm and ball from the surgical field. With known devices, any attempt to move the articulated arm and ball results in a loss of the desired trajectory that had been previously determined, thus requiring that the surgeon repeat the alignment process by which the desired trajectory was originally determined.
Therefore, there is a need for an improved surgical instrument support that can be moved from the surgical field and subsequently returned to its initial position without losing a previously established desired trajectory with respect to an intracranial target point.